In Delphi how I can set a TPanel variable type to null?

Question

Helo, I have the following stuff: selectedPanel should get the clicked panel object, and if the form is clicked, selectedPanel should be "null", nada, empty, etc :)

var
     selectedBlock: Tpanel; <== fixed typo

...

procedure TForm1.stubPanelMouseDown(Sender: TObject...
begin

    ...

    Panel:= Sender as TPanel;

    if (*selectedBlock is not null*) then
    begin
            // ie, store the current panel
            selectedBlock:= Panel;
    end
    else
    begin
            // empty the selection
            *selectedBlock:= null*;
    end;

So the question is: how I set that variable to "null"? Doing selectedBlock:= Unassigned throws me an error.

Thanks

EDIT: this still throws an error: access violation

if (selectedBlock=nil) then <= fixed and works
  begin
    selectedBlock:= Panel;
  end
  else
  begin
     selectedBlock:= nil;
 end;

Answer 1

Pointers are set to "null" using the nil constant:

selectedBlock := nil;

Null is a special value that only applies to Variant and OleVariant.